Thanks for passing along the tip, Dave. I looked up "Interpolation" in the manual, it is apparently a shortcut to reduce render time, so it makes sense that turning it off is helpful.

The artifacts are still there, but greatly reduced. The image above is of the worst out of 108 frames.The direct lighting in your scene is probably washing out what's left of the artifacts. My scene is a deliberate I-L torture test.

One more thing about these artifacts, they are VERY annoyingly random. Two successive renders of the same scene with exactly the same setting yield different illumination levels from I-L and may or may not contain visible artifacts. My animation's I-L still flickers like a fire, and the dance of the rectangles goes on.

I really admire what Carrara can do, but it's just THIS close to being perfect for what I have in mind... :-(